High power gyro devices, such as gyrotrons, gyroklystrons and gyro travelling wave tubes, are microwave vacuum tubes based on interaction between a helical electron beam having angular velocities and an electromagnetic field. The angular velocities are imposed by a DC magnetic field and are modulated as the beam passes through an oscillating electric field of a cavity or waveguide so that a high power electromagnetic wave is established in the region as a result of an interaction between the beam and field. The wave and beam travel along the same longitudinal axis while they are in the region. The periodic interaction between the beam and the field enables the beam and microwave circuit dimensions to be relatively large compared to a wavelength, whereby power density problems encountered in conventional millimeter wavelength travelling wave tubes and klystrons are avoided. The gyro devices are capable of developing extremely high, continuous wave power, such as 200 kilowatts, at millimeter wave frequencies, such as 28 GHz. Prior art references disclosing various facets of high power gyro devices are:
V. A. Flyagin et al., "The Gyrotron," IEEE Trans. MTT-25, No. 6, pp. 514-521, June 1977. PA1 J. L. Hirshfield and V. L. Granatstein, "The Electron Cyclotron Maser--An Historical Survey," IEEE Trans. MTT-25, No. 6, pp. 522-527, June 1977. PA1 N. I. Zaytsev, T. B. Pankratova, M. I. Petilin, and V. A. Flyagin, "Millimeter and Submillimeter Waveband Gyrotrons," Radiotekhnika i Elektronika, Vol. 19, No. 5, pp. 1056-1060, 1974. PA1 V. L. Granatstein, P. Sprangle, M. Herndon, R. K. Parker and S. P. Schlesinger, "Microwave Amplification with an Intense Relativistic Electron Beam," Journal of Applied Physics, Vol. 46, No. 9, pp. 3800-3805, Sept. 1975. PA1 P. Sprangle and A. T. Drobot, "The Linear and Self-Consistent Nonlinear Theory of the Electron Cyclotron Maser Instability," IEEE Trans. MTT-25, No. 6, pp. 528-544, June 1977. PA1 R. S. Symons and H. R. Jory, "Small-signal Theory of Gyrotrons and Gyroklystrons," 7th Symposium on Engineering Problems of Fusion Research, 1 Knoxville, TN, Oct. 1977. PA1 H. R. Jory, F. I. Friedlander, S. J. Hegji, J. F. Shively, and R. S. Symons, "Gyrotrons for High Power Millimeter Wave Generation," 7th Symposium of Engineering Problems of Fusion Research, Knoxville, TN, Oct. 1977.
In the prior art, it has been the practice to extract the millimeter wave energy coaxially with the beam axis. Hence, it is necessary for the millimeter wave energy to pass through an electron beam collector region prior to being supplied to an output waveguide of the high powered gyro device. However, when a continuous wave high power gyro device is operated so that 200 kilowatts are extracted from the millimeter wave, a collector for the electron beam must have a relatively large surface area. If the collector does not have a significant surface area, the electron beam power causes collector overheating, and possible destruction thereof. To achieve the large collector surface area, the collector must have a relatively large diameter. The wave must pass through the large diameter collector. To couple the wave to an output waveguide, it is necessary to have a tapered waveguide transition down to a smaller diameter, cylindrical output waveguide. The tapered waveguide transition to the cylindrical output waveguide causes higher order mode resonances in the collector. The portion of the millimeter wave power converted by the tapered waveguide to higher order electromagnetic modes cannot propagate in the output waveguide. Because these higher modes cannot propagate in the output waveguide, they become trapped in the collector vicinity. Resonances of the trapped modes in the collector vicinity occur as a function of frequency and collector dimensions. The resonances produce strong microwave reflections into the interaction region which interfere with the conversion of energy from the electron beam to the electromagnetic fields. Because of the limitations on the size of collectors which could be used on gyro devices as a result of the aforementioned problem with reflections, gyro devices have heretofore been limited to average power output in the order of several tens of kilowatts.